1. Field of the Invention
The present invention relates to a method for manufacturing a field emission display (FED), and more particularly to a method for manufacturing an FED having a double gate structure.
2. Related Art
Electronic displays are widely used for PC monitors, televisions (TVs) and other electrical products. These displays can be classified into cathode ray tubes (CRTs) and flat panel displays. Flat panel displays include liquid crystal displays (LCDs), plasma display panels (PDPs), and field emission displays (FEDs).
Field emission displays (FEDs) create a strong electrical field to extract electrons from emitters of a cathode electrode. The electrons collide with a phosphor material, whereby light emits from a desired pixel. A typical conventional field emission display comprises a plurality of emitters, with a micro-tip made of a metal such as molybdenum (Mo) being used as each emitter. With the development of nano-technologies, carbon nanotubes are widely used as emitters for FEDs. Such FEDs are sometimes known as carbon nanotube-based FEDs (CNT-FEDs). Compared to conventional technologies, e.g., cathode-ray tube (CRT) and liquid crystal display (LCD) technologies, CNT-FEDs have the advantages of a wide range of vision, high resolution, low energy consumption, smaller size, and good temperature stability.
Carbon nanotubes are very small tube-shaped structures essentially having a composition of a graphite sheet rolled into a tube. Carbon nanotubes produced by arc discharge between graphite rods were first discovered and reported in an article by Sumio Iijima entitled “Helical Microtubules of Graphitic Carbon” (Nature, Vol. 354, Nov. 7, 1991, pp. 56-58). Carbon nanotubes can have extremely high electrical conductivity, very small diameters (much less than 100 nanometers), large aspect ratios (i.e. length/diameter ratios) (greater than 1000), and a tip-surface area near the theoretical limit (the smaller the tip-surface area, the more concentrated the electric field, and the greater the field enhancement factor). Thus carbon nanotubes can transmit an extremely high electrical current, and have a very low turn-on electric field (approximately 2 volts/micron) for emitting electrons. For these reasons, carbon nanotubes have become of the most favored candidates for electrons emitters for electron emission devices, and now play an important role in field emission display applications. With the development of various different manufacturing technologies for carbon nanotubes, the research of carbon nanotube-based FEDs has yielded promising results.
Generally, FEDs can be roughly classified into diode type structures and triode type structures. Diode type structures have only two electrodes, a cathode electrode and an anode electrode. Diode type structures are unsuitable for applications requiring high resolution displays, because a typical diode type structure requires high voltages, produces relatively non-uniform electron emissions, and requires relatively costly driving circuits. Triode type structures were developed from diode type structures by adding a gate electrode for controlling electron emission. Triode type structures can emit electrons at relatively lower voltages, and can precisely control the emitted electrons to arrive at desired positions.
In order to realize better display quality, double gate-structured FEDs which have two gate electrodes have been developed. A conventional double gate-structured FED includes a substrate, a cathode layer formed on the substrate, a first insulating layer formed on the cathode layer, a first gate electrode formed on the first insulating layer, a second insulating layer formed on the first gate electrode, a second gate electrode formed on the second insulating layer, and an anode structure spaced from the cathode layer. The first insulating layer and the second insulating layer have a plurality of through holes for exposing the cathode layer. A plurality of emitters is provided on the exposed cathode layer, and electrons emitted from the emitters can travel through the through holes and arrive at a corresponding pixel. The second gate electrode of the conventional double gate-structured FED functions as a voltage control electrode or an electron focusing electrode, which facilitates a lower voltage threshold and focuses electrons toward to a corresponding pixel for high resolution display.
A conventional method for fabricating the above-described double gate-structured FED includes the steps of: forming a cathode layer on a substrate; forming a first insulating layer on the cathode layer; forming a first gate electrode with a first gate hole on the first insulating layer; forming a second insulating layer on the first insulating layer and the first gate electrode; forming a second gate electrode with a second gate hole on the second insulating layer; etching the second insulating layer through the second gate hole and the first insulating layer under the second insulating layer, and forming a through hole through which part of the cathode layer is exposed; forming a plurality of emitters on the exposed cathode layer; and assembling the obtained structure with an anode structure having a phosphor layer.
However, the above-described method may result in the following problems: first, the cathode layer is liable to be damaged during the etching step; second, the diameters of the through holes made by etching process are hard to control; and third, it is difficult to make the emitters in the through holes. Further, the structure of the insulating layers and the gate electrodes may be deformed or damaged during the fabricating of carbon nanotubes as the emitters, because of the high growth temperatures required.
Another conventional method for making the above-described double gate-structured FEDs includes the following steps: forming a cathode module which includes a substrate, a cathode layer being formed on the substrate and a plurality of emitters being formed on the cathode layer; forming a first gate module which includes a first insulation layer, a first gate layer, and a plurality of first through holes defined therein; forming a second gate module which includes a second insulation layer, a second gate layer and a plurality of second through holes defined therein; providing an anode module having a phosphor layer and an anode electrode attached on a transparent glass plate; assembling the first gate module and the second gate module with the cathode module; and assembling the anode module with the above-described obtained structure.
The above-described method is convenient for forming emitters, such as carbon nanotubes, on the cathode layer. Other modules are not deformed or damaged during the formation of the cathode module. However, the method is disadvantageous in that the first and the second gate modules need two alignments during assembling with the cathode module. The alignment of the two modules with very fine through holes is very difficult. Any mismatch or offset of the through holes of the two gate modules is liable to result in at least some of the electrons emitting from the emitters being blocked and not reaching the corresponding pixel. The quality of the display of the FED is diminished, or the display may even fail to function altogether.
Accordingly, what is needed is a method for manufacturing double gate-structured FEDs which overcomes the above disadvantages, eliminates mismatches, and promotes precision and high quality in mass production.